Process for polymerization of vinyl chloride

ABSTRACT

The dialkyl peroxydicarbonates with short alkyl chains, preferably diethyl and diisopropyl peroxydicarbonates, are used for the aqueous suspension polymerization of vinyl chloride, in the form of a solution in a dialkyl alkanedicarboxylate which is liquid and insoluble in water. The preferred solvents are hexanedicarboxylates (adipates) derived from adipic acid and from C 6 -C 10  alkanols. The peroxydicarbonate concentration of the said solutions is generally 15-40% by weight. The process according to the invention produces vinyl chloride polymers of improved quality resulting in shaped articles exhibiting markedly fewer fisheyes.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Ser. No. 09/117,098 filed Apr.23, 1999, now U.S. Pat. No. 6,258,906, which is the National Phase ofPCT Application PCT/EP97/00164 filed Jan. 10, 1997 which claims priorityfrom Belgium Application 96/000707 filed Jan. 25, 1996.

The present invention relates to a process for the aqueous suspensionpolymerization of vinyl chloride with the use of dialkylperoxydicarbonates. It relates more particularly to such a process inwhich dialkyl peroxydicarbonates with short alkyl chains are used in theform of a solution. The invention also relates to a process for themanufacture of a solution of dialkyl peroxydicarbonate with short alkylchains.

It is known to make use of dialkyl peroxydicarbonates for initiating theaqueous suspension polymerization of vinyl chloride. Dialkylperoxydicarbonates with short alkyl chains, such as diethyl anddiisopropyl peroxydicarbonates, constitute initiators that areparticularly appreciated because of their high activity at the usualtemperatures of polymerization of vinyl chloride. However, they have thedisadvantage of being unstable, with the result that their storage inthe pure state presents very serious hazards.

With a view to overcoming this disadvantage it has already been proposedto manufacture these peroxydicarbonates in the polymerization reactor(“in situ”), for example by reacting alkyl haloformate dissolved invinyl chloride with a peroxy compound such as hydrogen peroxide,dissolved in alkaline water. This process for “in situ” manufacture ofthe initiator does not allow an automation of the initiator feed to thepolymerization reactors. In addition, it lacks reproducibility (lack ofaccuracy concerning the quantities of initiator actually introduced intothe polymerization) and of production efficiency (need to precede eachpolymerization cycle with the “in situ” synthesis of the initiator).

It has also been proposed to prepare the quantity of dialkylperoxydicarbonate which is precisely needed, outside the polymerizationreactor (“ex situ”) and immediately before the polymerization.

This preparation is performed by reacting an alkyl haloformate with aperoxy compound in the presence of water and of a water-immisciblevolatile solvent which preferably has a boiling temperature lower than100° C., such as pentane or hexane. The initiator solution thus obtainedis then introduced in toto (organic phase and aqueous phase) into thepolymerization reactor which is subsequently charged with a view to thepolymerization (British Patent 1 484 675 in the name of Solvay & Cie).This process allows the initiator feed to the reactors to be automatedbut still makes it necessary to produce the sufficient precise quantityof initiator immediately before the polymerization. Besides, it does notallow (either) a delayed introduction of the dialkyl peroxydicarbonates,a technique that is advantageous, for example, in order to improve thepolymerization kinetics. In addition, just like the abovementionedprocess for “in situ” manufacture, it produces vinyl chloride polymerswhich, after conversion, result in finished articles containing many“fisheyes”.

British Patent Application 2 022 104 and French Patent Application 2 352839 mention processes for the aqueous suspension polymerization of vinylchloride with the use of dialkyl peroxydicarbonates with short alkylchains in the presence of respectively a plasticizer or a diacid higheralcohol ester. The processes described in these documents do however notallow to solve the problems linked with the dialkyl peroxydicarbonateinstability during storage, the automated initiator feed to thepolymerization reactors and the delayed introduction in thepolymerization reactor of these initiators.

The U.S. Pat. No. 3,950,375 relates to a continuous process for themanufacture of pure dialkyl peroxydicarbonates by centrifuging theaqueous reaction phase.

The U.S. Pat. No. 3,377,373 describes a continuous process for themanufacture of a diisopropyl peroxydicarbonate solution in carbontetrachloride.

The aim of the present invention is to provide a process for the aqueoussuspension polymerization of vinyl chloride with the use of dialkylperoxydicarbonates with short alkyl chains which does not exhibit any ofthe abovementioned disadvantages. It also aims to provide an improvedprocess for the manufacture of a solution of dialkyl peroxydicarbonateswhich are particularly suited for use in the aqueous suspensionpolymerization of vinyl chloride.

To this end, the invention relates to a process for the aqueoussuspension polymerization of vinyl chloride with the use of dialkylperoxydicarbonates with short alkyl chains, characterized in that thedialkyl peroxydicarbonate is used in the form of a solution in a dialkylalkanedicarboxylate which is liquid and insoluble in water.

The solution of dialkyl peroxydicarbonate with short alkyl chains whichis used according to the process of the present invention consistsessentially of dialkyl peroxydicarbonate and of solvent (dialkylalkanedicarboxylate). It is therefore free from other polymerizationingredients such as, for example, monomer.

Dialkyl alkanedicarboxylate (hereinafter referred to briefly as “ester”)which is liquid and insoluble in water is intended to denote the esterswhich are liquid and insoluble in water in normal conditions, that is tosay at ambient temperature and at atmospheric pressure. Insoluble inwater is intended more particularly to mean a solubility in water atambient temperature which is lower than 0.5 g/l. The solubility in waterof the esters used as solvent for the peroxydicarbonate in the processof the invention preferably does not exceed 0.3 g/l.

The esters which are liquid and insoluble in water and used in theprocess of the invention generally have boiling temperatures (in normalconditions) which are appreciably higher than 100° C. In most cases theyare higher than 150° C.

By way of examples of esters that can be applied there may be mentionedthe liquid and water-insoluble esters as defined above which are derivedfrom C₄-C₁₀ alkanedicarboxylic acids and from C₂-C₁₂ alkanols (linear orbranched saturated aliphatic alcohols). Examples of these that may bementioned are diethyl and dibutyl butanedicarboxylates (succinates),diethyl, dipropyl, dibutyl, diisobutyl and diethylhexylhexanedicarboxylates (adipates), diethyl and dibutyloctanedicarboxylates (suberates) and dibutyl, diethylbutyl anddiethylhexyl decanedicarboxylates (sebacates).

Esters which are well-suited for carrying out the process of theinvention are the alkanedicarboxylates derived from C₄-C₈alkanedicarboxylic acids and from C₆-C₁₀ alkanols. Esters which are veryparticularly preferred are chosen from hexanedicarboxylates (adipates)derived from adipic acid and from C₆-C₁₀ alkanols. An ester which isvery particularly preferred in the process of the invention isdiethylhexyl adipate.

The concentration of dialkyl peroxydicarbonate in the solutions used inthe polymerization process according to the invention is generally fromapproximately 15 to 40% by weight. The use of dilute peroxydicarbonatesolutions, for example of solutions containing approximately 10% byweight (or less) of dialkyl peroxydicarbonate, introduces the risk ofresulting in vinyl chloride polymers whose glass transition temperature,and hence heat resistance, is reduced. In general, approximately 40% byweight is not exceeded because a concentration that is too high reducesthe accuracy of the measurement when the reactor is fed with initiator.Good results are obtained with solutions in which the dialkylperoxydicarbonate concentration is from approximately 25 to 35% byweight.

The solutions of dialkyl peroxydicarbonates with short alkyl chains thatare used in the polymerization process according to the invention may bestored without risk at low temperature (below 10° C.) and this may bedone for many hours without appreciable loss of activity. They canconsequently be prepared in advance in a sufficient quantity to feed anumber of polymerization reactors or else to feed a number ofpolymerization cycles in the same reactor.

For the purpose of the present invention, dialkyl peroxydicarbonateswith short alkyl chains are intended to denote the peroxydicarbonates inwhich the alkyl radicals contain 2 or 3 carbon atoms and representethyl, propyl or isopropyl radicals, more particularly the ethyl andisopropyl radicals. A peroxydicarbonate which is very particularlypreferred is diethyl peroxydicarbonate.

According to a particularly preferred embodiment of the process of theinvention a diethyl or diisopropyl peroxydicarbonate is hence used inthe form of a solution in a hexanedicarboxylate (adipate) derived fromadipic acid and from a C₆-C₁₀ alkanol.

It is to be understood that, besides the dialkyl peroxydicarbonates withshort alkyl chains, other conventional initiators may be used conjointlyin the polymerization process of the invention. As examples of suchother initiators there may be mentioned dilauroyl and dibenzoylperoxides, azo compounds or dialkyl peroxydicarbonates with long alkylchains, such as dicetyl peroxydicarbonate. Nevertheless, it is preferredto initiate the polymerization exclusively with the use of dialkylperoxydicarbonates with short alkyl chains. In contrast to the otherabovementioned peroxides, these have the advantage that their residuesor excesses that may be present in polymerization mixture at the end ofthe polymerization cycle (and that could affect the thermal stability ofthe vinyl chloride polymers resulting from the process) are easilydestroyed simply by alkalifying the mixture at the end of thepolymerization cycle.

It is also to be understood that the dialkyl peroxydicarbonates inorganic solution can be introduced, wholly or partially, after thebeginning of the polymerization (with a delay). The delayed use of aportion of the dialkyl peroxydicarbonate with short alkyl chains isadvantageous in order to improve the polymerization kinetics or else inorder to produce resins with a low K value (which are produced atelevated temperature) exhibiting a good heat stability. The totalquantity of initiator used generally ranges from approximately 0.15 to0.90, and still more particularly from approximately 0.20 to 0.35 partsper thousand by weight approximately relative to the monomer(s) used.

Apart from the particular feature of the use of a dialkylperoxydicarbonate with short alkyl chains (C₂ or C₃) in the form of asolution in an ester, the general polymerization conditions are thoseusually used for the noncontinuous polymerization of vinyl chloride inaqueous suspension.

For the purpose of the present invention vinyl chloride polymerizationis intended to denote both the homopolymerization of vinyl chloride andits copolymerization with other ethylenically unsaturated monomers thatcan be polymerized by a radical route. Examples of conventionalcomonomers of vinyl chloride that can be used in the process of theinvention and which may be mentioned are olefins, halogenated olefins,vinyl ethers, vinyl esters such as, for example, vinyl acetate, andacrylic esters, nitriles and amides. The comonomers are used inquantities which do not exceed 50 mol %, in most cases 35 mol %, of themixture of comonomers used. The process according to the invention ishighly suited for the homopolymerization of vinyl chloride.

Aqueous suspension polymerization is intended to mean the polymerizationwith the use of oil-soluble initiators, in this case especially dialkylperoxydicarbonates with short alkyl chains, in the presence ofdispersing agents such as, for example, water-soluble cellulose ethers,partially saponified polyvinyl acetates (also called polyvinyl alcohols)and mixtures thereof. Surface-active agents can also be used at the sametime as the dispersing agents. The quantity of dispersing agent usedgenerally varies between 0.7 and 2.0 parts per thousand by weightrelative to the monomer(s).

The polymerization temperature is usually between approximately 40 and80° C.

At the end of polymerization the vinyl chloride polymers producedaccording to the process of the invention are isolated in a conventionalmanner from their polymerization medium, generally after having beensubjected to a purification from residual monomer(s).

The polymerization process of the invention allows an automation of thereactor feed. It results in an improvement in the reproducibility of thepolymerization cycles. Furthermore, the use of the dialkylperoxydicarbonates in the form of a solution in an ester according tothe invention does not significantly affect the polymerization kineticsor the general properties (such as the K value, density and particlesize) of the vinyl chloride polymers produced. In addition, whenmelt-processed, the latter provide shaped articles exhibiting a muchsmaller number of fisheyes.

The present invention also relates to an improved process for thetwo-stage manufacture of a solution of dialkyl peroxydicarbonate withshort alkyl chains capable of being employed in (and particularly suitedfor) the aqueous suspension polymerization of vinyl chloride.

According to this process, in a first stage, a dialkyl peroxydicarbonatewith short alkyl chains (as defined above) is manufactured by reactingappropriate quantities of alkyl haloformate with an inorganic peroxidein water in the presence of an inorganic salt in sufficient quantity toincrease the density of the aqueous reaction medium and, in a secondstage, the dialkyl peroxydicarbonate manufactured is isolated byextraction by means of a water-insoluble solvent, to produce a solutionof dialkyl peroxydicarbonate in this, solvent.

The inorganic salt is advantageously used in a sufficient quantity tobring the density of the aqueous reaction medium to a value of at least1.05 and still more particularly to a value of at least 1.10.Furthermore, it is appropriate to adapt the quantity of inorganic saltso that it does not exceed the salt saturation concentration of theaqueous reaction medium.

The nature of the salt used in the stage of the manufacture of thedialkyl peroxydicarbonate is not particularly critical. In principle anyinorganic salt that does not interfere with the reaction of formation ofthe dialkyl peroxydicarbonate, and which does not precipitate in thereaction conditions, is suitable. For example halides may be mentionedas nonlimiting examples of such salts, and in particular alkali andalkaline-earth metal chlorides. Alkali metal chlorides are preferablyemployed. According to a particularly advantageous embodiment, sodiumchloride is employed.

The fact of carrying out the manufacture of the peroxydicarbonate in adensified aqueous medium results in the end in improving the efficiencyof the isolation of the dialkyl peroxydicarbonate in solution.

The essential particular feature of the first stage is the use of aninorganic salt in sufficient quantity to increase the density of theaqueous reaction phase.

In most cases the reaction temperature is at a value situated between−10° C. and +10° C. The manufacture of the peroxydicarbonate isgenerally complete after a few minutes' reaction; the reaction periodgenerally does not exceed 10 minutes and in most cases 5 minutes.

The alkyl haloformate is in most cases and advantageously achloroformate. The inorganic peroxide is in most cases calcium or sodiumperoxide or else aqueous hydrogen peroxide. In this latter case it isappropriate, in addition, to introduce a base, such as calcium hydroxideor sodium hydroxide, into the aqueous reaction medium.

It is particularly advantageous to use alkyl chloroformate with sodiumperoxide or else hydrogen peroxide in the presence of sodium hydroxideas base (which leads to the formation of sodium chloride as byproduct),and, furthermore, to use sodium chloride as inorganic salt fordensifying the aqueous phase. In this case the saline aqueous phasesubsequently recovered (after isolation of the dialkyl peroxydicarbonatesolution by extraction) can, without disadvantage, be recycled(optionally after dilution) to the manufacture of a new quantity ofdialkyl peroxydicarbonate solution.

This procedure has the twin advantage of substantially reducing theusage of inorganic salt for densifying the aqueous phase and ofreducing, or even eliminating, the environmental problems related to theremoval of the saline aqueous phase after the manufacture of the dialkylperoxydicarbonate.

The nature of the water-insoluble solvent employed in the second stagefor the extraction of the dialkyl peroxydicarbonate is not particularlycritical. A water-insoluble solvent is intended to denote a solventwhich is insoluble in water at ambient temperature and atmosphericpressure and, more particularly, a solvent whose solubility in water inthese conditions is lower than 0.5 g/l and still more particularly lowerthan 0.3 g/l.

By way of nonlimiting examples of solvents that can be employed for theextraction of the dialkyl peroxydicarbonate there may be mentioned thewater-insoluble organic compounds chosen from the usual plasticizers forpolyvinyl chloride. Nonlimiting examples of such solvents which may bementioned are esters of aromatic polycarboxylic acids (like dibutyl ordiethylhexyl phthalates), alkyl epoxycarboxylates (like octylepoxystearate), epoxidized oils (like epoxidized soya oil) or thedialkyl alkanedicarboxylates the definition of which is given above inthe context of the description of the dialkyl peroxydicarbonatesolutions used in the aqueous suspension polymerization of vinylchloride.

It is particularly advantageous to choose a solvent which furthermorehas a relative density lower than 1 and preferably lower than 0.95.

Solvents that are particularly preferred are chosen from dialkylalkanedicarboxylates derived from C₄-C₈ alkanedicarboxylic acids andfrom C₆-C₁₀ alkanols. Solvents that are very particularly preferred arechosen from the hexanedicarboxylates (adipates) derived from adipic acidand from C₆-C₁₀ alkanols. Excellent results are obtained withdiethylhexyl adipate (boiling temperature at atmospheric pressure: 214°C., solubility in water at ambient temperature: <0.2 g/l, density:0.922).

The quantity of solvent employed for the extraction is not critical. Itis obvious that it will depend especially on the degree of solubility ofthe dialkyl peroxydicarbonate in the solvent chosen. This quantity willadvantageously be such that the final concentration of the dialkylperoxydicarbonate solution is from approximately 15 to approximately 40%by weight and still more particularly from 25 to 35% by weight.

The second stage of manufacture of the dialkyl peroxydicarbonatesolutions, namely the isolation by extraction of the dialkylperoxydicarbonate manufactured in the first stage, is performed in anyknown and appropriate manner.

The extraction solvent is advantageously added to the aqueous reactionmixture after the reaction of manufacture of the dialkylperoxydicarbonate is finished, the phases are allowed to settle out andthe supernatant organic phase is separated from the aqueous reactionphase in order to collect a pure peroxydicarbonate solution.

It is imperative to add the extraction solvent to the aqueous reactionmixture only after the end of the reaction of formation of theperoxydicarbonate. It has been found, in fact, that when the solvent ispresent from the beginning of the reaction, its presence has the effectof slowing the reaction down and of affecting the purity of theperoxydicarbonate solutions finally produced. In practice, therefore,the addition of the solvent will take place at the earliestapproximately 5 minutes after the beginning of the reaction.

According to a very particularly preferred and advantageous embodiment,a solution of dialkyl peroxydicarbonate with short alkyl chains (such asdiethyl, dipropyl or diisopropyl) containing from 15 to 40% by weight ofdialkyl peroxydicarbonate is manufactured by using, in the first stageof the process of manufacture, sodium chloride as inorganic salt inorder to increase the density of the aqueous phase and, in the secondstage, of C₆-C₁₀-alkanol adipates, in particular diethylhexyl adipate,as extraction solvent for producing a solution of dialkylperoxydicarbonate.

The invention also relates to a process for the manufacture of asolution of dialkyl peroxydicarbonate in which the alkyl radicalscontain 2 or 3 carbon atoms according to which, in a first stage adialkyl peroxydicarbonate in which the alkyl radicals contain 2 or 3carbon atoms is manufactured by reacting, in water, appropriatequantities of alkyl haloformate with an inorganic peroxyde in thepresence of an inorganic salt in sufficient quantity to increase thedensity of the aqueous reaction mixture and, in a second stage, thedialkyl peroxydicarbonate manufactured is isolated by extraction bymeans of a water-insoluble solvent, chosen from the water-insolubleorganic compounds chosen from the usual plasticizers for polyvinylchloride, in order to produce a solution of dialkyl peroxydicarbonate inthis solvent.

The process of manufacture of dialkyl peroxydicarbonate solutionsaccording to the invention provides solutions that are pure and stablein storage, in high yields. These solutions can be conveyed withoutdanger and do not give rise to problems of deposits in conduits.

EXAMPLE 1

The example which follows is intended to illustrate the invention. Itrelates to the aqueous suspension homopolymerization of vinyl chloridewith the use of diethyl peroxydicarbonate in solution containingapproximately 30% by weight in diethylhexyl adipate. Theperoxydicarbonate is manufactured from ethyl chloroformate, hydrogenperoxide and sodium hydroxide, before being extracted with diethylhexyladipate.

Preparation of the solution of diethyl peroxydicarbonate

Into a 1000-l stirred reactor cooled below 10° C. are introduced 622 kgof an aqueous solution of sodium chloride containing 180 g/kg (that is510 kg of demineralized water and 112 kg of NaCl), precooled to 5° C.20.4 kg of ethyl chloroformate and 8.5 kg of aqueous solution ofhydrogen peroxide containing 350 g/kg are then introduced successivelyinto the stirred aqueous solution, and finally, very slowly, 36.1 l ofaqueous solution of sodium hydroxide containing 200 g/kg, so as tomaintain the temperature below 10° C. The density of the aqueousreaction mixture rises to 1.11. 10 minutes after the end of theintroduction of the NaOH solution 34.5 kg of diethylhexyl adipate areintroduced, precooled to 5° C. After the reaction mixture has been keptstirred for 15 minutes while being cooled to 5° C., the stirring isstopped. The aqueous phase (dense phase) is then separated off aftersettling out and the organic phase is recovered. The solution of diethylperoxydicarbonate in diethylhexyl adipate which is thus produced isstored at 5° C. with a view to its subsequent use. Its diethylperoxydicarbonate content (determined by analysis) is 287 g/kg.

Vinyl chloride polymerization

Into a reactor with a capacity of 3.9 m³, equipped with a stirrer and ajacket, are introduced at ambient temperature and with stirring (50rev/min) 1869 kg of demineralized water, 0.801 kg of polyvinyl alcohol(degree of hydrolysis 72 mol %) and 0.534 kg of polyvinyl alcohol(degree of hydrolysis 55 mol %), and 1.793 kg of the initiator solutionprepared as above (that is 0.515 kg of diethyl peroxydicarbonate). Thereactor is closed, the stirring is stopped and the reactor is placedunder partial vacuum (60 mm Hg absolute), which is maintained for 5minutes. The stirrer is restarted (110 rev/min) and 1335 kg of vinylchloride are then introduced. The mixture is heated to 53° C., afterwhich cold water is circulated through the jacket. The instant when thepolymerization mixture reaches 53° C. is considered to be the beginningof the polymerization (time=t₀). After 6 h of running (counted from t₀)the pressure in the reactor has dropped by 1.5 kg/cm². Thepolymerization is stopped by successively performing an introduction of0.35 kg of ammonia, the degassing of the unconverted vinyl chloride andcooling. The polyvinyl chloride produced is isolated from the aqueoussuspension in conventional manner. 1118 kg of PVC are collected, the Kvalue of which (at 20° C. in cyclohexanone at a concentration of 5 g/l)is 71.0.

The table below summarizes the properties evaluated on the PVC produced:K value (at 20° C. in cyclohexanone at a concentration of 5 g/l),apparent density (AD), porosity (% absorption of diethylhexylphthalate), particle size distribution and, finally, the number offisheyes, expressed as points per dm and evaluated on a film extrudedstarting with a mixture of 100 parts by weight of PVC and 40 parts ofdiethylhexyl phthalate.

EXAMPLE 2 (Comparative)

By way of comparison, the vinyl chloride polymerization was reproducedin the same conditions as in Example 1, except that the appropriatequantity of diethyl peroxydicarbonate was first synthesized in situ inthe polymerization reactor by reacting, at ambient temperature and withstirring, 0.734 kg of ethyl chloroformate and 0.109 kg of hydrogenperoxide in the presence of the total quantity of water (alkalified byaddition of 0.284 kg of sodium hydroxide) and of the total quantity ofpolyvinyl alcohols intended for the polymerization (cf. Example 1: i.e.1860 kg of water and, in all, 1.335 kg of polyvinyl alcohols). At theend of “in situ” synthesis of the initiator the reactor is closed, thestirring is stopped and the reactor is placed under partial vacuum (60mm Hg absolute) for 5 minutes and 1335 kg of vinyl chloride areintroduced with stirring (110 rev/min). The heating and thepolymerization are subsequently carried out as in Example 1. After 5 h51 min the pressure in the reactor has dropped by 1.5 kg/cm and thepolymerization is stopped. 1092 kg of PVC are collected, the K value ofwhich (measured in the same conditions) is 71.3.

The table below also summarizes the properties evaluated on the PVCproduced according to comparative Example 2.

From the comparison of the results shown in the table it appears thatthe use of diethyl peroxydicarbonate in solution of diethylhexyl adipate(according to the invention) has no significant effect on thepolymerization kinetics or on the general properties of the PVCproduced. In addition, the film extruded by starting from the PVCproduced according to the invention (Example 1) has a significantlyreduced number of fisheyes by comparison with a film extruded bystarting from PVC produced with the use of diethyl peroxydicarbonateproduced “in situ” (comparative Example 2).

TABLE Example number 1 2 Total polymerization period, h min 6.00 5.51 Kvalue 71.0 71.3 AD, kg/1 0.484 0.486 Porosity, % 33.3 32.5 Particle sizedistribution, g/kg >250 μm 177-250 μm 4 5 125-177 μm 57 72 88-125 μm 461507 63-88 μm 422 369 45-63 μm 54 46 <45 μm 2 1 0 0 Number of fisheyes,pts/dm² 8 44

What is claimed:
 1. A process for aqueous suspension polymerization ofvinyl chloride, optionally in a mixture with comonomers, said processemploying at least one dialkyl peroxydicarbonate in which the alkylradicals contain 2 or 3 carbon atoms, wherein the dialkylperoxydicarbonate is provided in the form of a solution consistingessentially of the dialkyl peroxydicarbonate in a dialkylalkanedicarboxylate which is liquid and insoluble in water.
 2. Theprocess of claim 1, wherein said comonomers comprise less than 50 mol %of said mixture.
 3. The process of claim 1, wherein the dialkylalkanedicarboxylate is a liquid ester derived from C₄-C₁₀alkanedicarboxylic acids and from C₂-C₁₂.
 4. The process of claim 3,wherein the dialkyl alkanedicarboxylate is a hexanedicarboxylateesterified with C₆-C₁₀ alkanols.
 5. The process of claim 1, wherein theconcentration of dialkyl peroxydicarbonate in the solution is from 15 to40% by weight.
 6. The process of claim 1, wherein the dialkylperoxydicarbonate is diethyl or diisopropyl peroxydicarbonate and thedialkyl alkane dicarboxylate is the hexanedicarboxylate of C₆-C₁₀alkanol.
 7. The process of claim 1, wherein said dialkylperoxydicarbonate in which the alkyl radicals contain 2 or 3 carbonatoms is the sole initiator of polymerization.